In today's devices, interfaces become more and more important. They shall be reliable and low power. More and more so-called soft-keys are used, where the software of the device does adapt the function of the button. One exception is the hardware reset, which has to function without any software interference to be used as a last resort if a device becomes non-responsive.
It has been known to use photodiodes as input elements for electronic devices. Typically, the input element has to be covered by the user in order to operate it, thereby reducing the current through the photodiode, which can be detected by suitable circuitry.